Electron paramagnetic resonance (EPR) allows spectroscopic analysis of substances based on physical concepts analogous to those used in nuclear magnetic resonance (NMR). While NMR allows analysis of substances containing nuclides with non-zero spin, EPR is only applicable to substances containing chemical agents that possess at least one unpaired electron. NMR proves particularly useful in the analysis of substances comprising hydrogen atoms, which are abundantly present in water and hydrocarbons. Furthermore, Magnetic Resonance Imaging (MRI), an imaging technique based on NMR, is a valuable tool in medical diagnosis, due to the subtle contrasts caused by water density and complex spin-spin and spin-lattice interactions in different tissues.
EPR, on the other hand, has found less application in the past because all electrons in most stable chemical compounds are paired. However, the strength of EPR lies in its high specificity. EPR can readily be used for detection and imaging of free radicals in tissues, but the development of specific spin-labeled biological tracer molecules has spawned opportunities for the usage of EPR, and particularly the usage of EPR-based imaging techniques, for analysis of diverse physiological functions in biology and medicine. This opens the way for new tracers, specific to biological mechanisms that can't be studied by conventional means, and for alternatives to tracers used in nuclear medicine, without the implied radiation exposure caused by radionuclides.
EPR typically uses DC magnetic fields of 5 mT to 1.25 T or higher to cause magnetic polarization of particles with non-zero electron spin. Narrow-band radio-frequent waves are used to disturb the magnetization and cause resonance. The frequency at which resonance occurs, referred to as the Larmor precession frequency, is dependent on the applied magnetic field strength and specific material properties, and can range from 200 MHz for low field strengths to 35 GHz or higher for strong fields. The low-field (<30 mT) low-frequency (<1 GHz) region is particularly of interest for applications in biology and medicine because of diminished dielectric loss in tissues.
Quantification of the amount of magnetic, e.g. paramagnetic, particles using EPR signals can be performed directly on the EPR signal obtained using conventional EPR measurements. Nevertheless, in order to deal with a wide range of concentrations and to perform accurate quantification, there is still room for improvement.